a. Technical Field
The present invention is in the field of non-volatile memory integrated circuits.
b. Description of the Related Art
Two well known types of non-volatile memory integrated circuits are: (1) electrically-erasable electrically-programmable read only memory (EEPROM) integrated circuits; and (2) electrically flash reprogrammable read only memory (flash) integrated circuits.
A typical EEPROM includes an array of memory cells, with each memory cell consisting of two MOSFET transistors: a select transistor and a storage transistor. The select transistor controls access to the storage transistor. The storage transistor includes a source region, a drain region, and a channel region between the source and drain regions. Two gates overlie the channel region: (1) a lowermost, electrically-isolated, floating gate; and (2) an overlying, control gate. A thin oxide layer, called tunnel oxide, is between the floating gate and the channel region. Electrons move back and forth through the tunnel oxide by Fowler-Nordheim tunneling, leaving the floating gate with either a net positive or a net negative charge. When a net positive charge is on the floating gate, the storage transistor conducts when a specified read voltages is applied to the control gate. When a net negative charge is on the floating gate, the storage transistor does not conduct upon application of the read voltage. The conductive state is interpreted as a logical one, and the nonconductive state is interpreted as a logical zero.
EEPROMs have attributes that make them better for some applications than others, due to the fact that EEPROMs have separate select and storage transistors. For instance, EEPROMs are robust and reliable. Moreover, because of the separate select transistor, EEPROM cells may be erased at the byte and page level. EEPROM cells also are efficient users of current, because the programming current is very low with Fowler-Nordheim tunneling. On the other hand, EEPROMs are relatively low speed, and each EEPROM cell occupies a relatively large area. The large area results from the presence of the two transistors, and the need for each memory cell to have contacts for connecting both to a bitline and a wordline.
A flash memory, by contrast to an EEPROM, is comprised of single transistor memory cells. The flash memory cell includes a lowermost, polysilicon floating gate, and an overlying polysilicon control gate. A thin tunnel oxide layer separates the floating gate from the substrate. Both the programming and erase operations occur through Fowler-Nordheim tunneling of electrons through the tunnel oxide between the floating gate and the semiconductor substrate.
Like EEPROMs, flash memory has features that make it better for some applications than others. For instance, flash memory cells occupy much less area than EEPROM cells, and are faster. However, flash memories cannot be erased in as selective a manner as an EEPROM. Flash memory is erased in blocks. Further, in a flash memory, because there is not a separate select transistor, an operation directed at one cell can easily disturb the stored charge on the floating gate of nearby cells. Because of this risk of disturbance, flash memories must include circuit to verify the contents of the memory. This verify circuitry consumes current, which can affect the operation time of battery-operated devices. In addition, while flash cells are much smaller than EEPROM cells, there are contacts at each memory cell to a bitline and a wordline, and these contacts consume valuable chip area.
Clearly, it would be desirable to have a non-volatile memory that combines the reliability and low current operation of an EEPROM, while at the same time having the small size and speed of a flash memory.